Solder formulations known as solder creams or solder pastes are homo-geneous blends of a soft solder alloy typically in a powder form dispersed in a liquid medium conventionally containing a fluxing composition or flux, an organic solvent, and a thickening agent which will give the desired viscous or paste-like consistency to the solder formulation. Such solder formulations can be applied to the surfaces or locations requiring soldering in a number of various ways, such as by screen printing, or by means of a dispenser such as a syringe, or simply by dipping the site to be soldered into the solder paste formulation so that the viscous paste adheres to the site, which may be an electronic component lead.
Recently, solder paste formulations have been used increasingly by the electronics industry, particularly in the automated manufacture of printed circuits in which leadless miniature electronic components are surface mounted on a printed circuit board (PCB) to which a solder paste formulation has previously been applied, such as by screen printing. The PCB is then subjected to a sufficiently high temperature, for example by means of a heated conveyor belt, to cause the flux and solder alloy in the formulation to liquefy and contact the electronic component leads so that on subsequent cooling of the PCB, the components will remain soldered on the PCB.
The rheological properties of the solder paste dictate the screen print-ability of the material. Hence, in order to obtain good line definition on the PCB, the rheology of the paste must fall within a relatively narrow process window. Solder paste rheology is largely determined by those components of the solder paste which are collectively known as the vehicle. Generally, solder paste vehicles consist of solvents to dissolve the rosin fluxes or fluxing agents and the activators, and other additives to achieve the proper rheology. Currently, cyclohexanol (sorbitol) derivatives and various Cellosolve.TM. mono- and dialkyl ethers of ethylene glycol and derivatives thereof are used in commercial solder pastes. Problems encountered with conventional solvents include poor solubility of the organic acid fluxing agents in the solvents which is sometimes addressed by using more solvent than is considered desirable, and an inability to obtain high viscosities (poor rheological control). Many of these components do not volatilize under reflow conditions and therefore result in a residue which needs to be removed by chlorofluorocarbon (CFC), terpene, aqueous, exotic gas or other cleaning techniques. Some of these materials are hazardous or toxic and CFCs have been linked to the undesirable degradation of atmospheric ozone. Thus, it would be an improvement in the art to eliminate residues and the materials used to remove them.
For some uses in the electronics industry, it is desirable to use as the flux composition of the solder formulation a material which is non-corrosive and which will leave, after the heating and cooling steps, flux residues which are themselves non-corrosive and non-conducting. For this reason, rosin-based flux compositions are widely used in the commercially available solder paste formulations specifically made for use in the manufacture of surface mounted electronic components.
Alternatively, more reactive fluxing compositions may be used, which leave residues which are corrosive and/or conductive. Often a somewhat corrosive fluxing composition is desired so that the oxides which form on the metal surfaces may be removed to permit the subsequently formed solder bond to be stronger both physically and electrically. However, it is necessary to remove the residues formed by means of either aqueous or organic solvent systems to ensure that the resulting soldered circuit is non-corrosive.
The use of solder paste formulations containing such rosin-based or more reactive fluxes produces a number of disadvantages. First, because non-corrosive residues tend to be sticky, they prevent repetitive automatic testing of the circuit. Additionally, such residues are unsightly and therefor, as with the corrosive flux residues which are also unattractive, will need to be removed. The removal step involves extra production equipment, time and material.
Secondly, flux residues tend to be hygroscopic and may thereby cause spattering. Thirdly, some fluxes permit solder particles in the paste to move away from the solder site and give rise to the formation of numbers of discrete small balls of soft solder around the soldered joints, which can create electrical short circuits.
Because of these and other disadvantages, it is desirable and often essential to meet specifications, to remove the flux residues and any solder balls as much as possible. Often, however, their removal is difficult or impossible, particularly from areas of the PCB underneath the electronic components. As noted, a common procedure is to use an aqueous or organic solvent in the removal of flux residues. Aqueous washes are preferred because the disposal of the waste products is easier than for organic solvents.
Though the solvents of the vehicles are usually readily removable by water or mild solvents, any new system should require no cleaning at all. It would therefore be advantageous to discover a new flux vehicle that would avoid one or more of the disadvantages of the prior solder flux vehicles.